The art of lithographic printing is based on the immiscibility of ink and water. A lithographic printing plate is composed of ink receptive regions, commonly referred to as the “image area,” and hydrophilic regions. When the surface of the printing plate is moistened with water and printing ink is applied, the hydrophilic regions retain the water and repel the printing ink, and the image area retains the printing ink and repels the water. The printing ink retained on the image area may then be transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the desired surface.
Lithographic printing plate precursors typically include a radiation-sensitive coating applied over the hydrophilic surface of a substrate. Conventional radiation-sensitive coatings include photosensitive components dispersed within an organic polymeric binder. After a portion of the coating is exposed to radiation (commonly referred to as imagewise exposure), the exposed portion becomes either more developable or less developable in a particular liquid than an unexposed portion of the coating. A printing plate precursor is generally considered a positive-working plate if, after exposure to radiation, the exposed portions or areas of the radiation-sensitive coating become more developable and are removed in the developing process to reveal the hydrophilic surface. Conversely, the precursor is considered a negative-working plate if the exposed portions or areas become less developable in the developer and the unexposed portions or areas are removed in the developing process.
After imaging, the precursors are contacted with a developer to remove either the exposed or unexposed portions of the radiation-sensitive coating to form a printing plate. This process is generally performed using a liquid developer system, which receives imaged printing plate precursors, and then contacts the imaged precursors with the liquid developer either by spraying the developer onto the imaged precursor or by immersing the imaged precursor in a developer bath. During development, portions of the radiation-sensitive coating are removed from the imaged precursor to reveal the substrate surface, and portions remain on the plate to provide an ink-receptive image. Oftentimes, the used developer is filtered and then reused to develop additional imaged precursors. Examples of developers used in such processes include 956 brand developer, MX 1813 brand developer and Aqua-Image brand developer, all of which are available from Kodak Polychrome Graphics, Norwalk, Conn.
During the development of a large number of printing plate precursors, the activity of a developer (i.e. the ability of the developer to remove desired portions of the radiation-sensitive coating to produce an image) may vary due to the depletion of, or changes in, various components of the developer. In one example, the concentration of the active solvent or the dispersing agent may become depleted. In another example, the pH of the developer may vary. Changes in developer activity may be caused by loss of developer volume as printing plates carry developer out of the developer apparatus. Additionally, interactions with the radiation-sensitive coatings of the printing plate precursors may also affect developer activity. In particular, the pH of the developer may change due to acid/base interactions with radiation-sensitive coatings. This loss of developer activity may result in inconsistency in overall dot density over a cycle of developed printing plates, which may have adverse effects during printing.
If the developer has a measurable electrical conductivity, as is usually the case with aqueous alkaline developers, users often monitor the electrical conductivity of the developer during the development process to determine the efficiency of the developer. As soon as a weakening of the developer efficiency is established by finding of a lower electrical conductivity, the developer is refreshed by adding either fresh developer or a replenisher composition (that may have a higher concentration of hydroxide ions than the developer) to the depleted or weakened developer until the electrical conductivity of the developer again reaches its original value.
As described in EP-A-0 556 690, the electrical conductivity of a developer does not always decrease when the alkalinity decreases during the development process. Depending on the kind of developer system and also on the kind of plate being developed, the electrical conductivity of the developer may be constant, may increase, or may decrease during use, even though the efficiency of the developer decreases in each case. According to one theory, it is assumed that ingredients of the removed radiation-sensitive layer of the developed printing plates and/or side reactions are responsible for the production of new conductive ions. These ions at least partially compensate or even overcompensate the loss of electrical conductivity due to consumption of hydroxide ions of the developer, thus producing a measured electrical conductivity that is not as low as it would be expected due to the consumption of hydroxide ions by the developing process. Measuring the electrical conductivity does not therefore allow a conclusion on the efficiency or activity of the developer in every case. Constant or unchanging conductivity values therefore do not guarantee the developer activity required for the desired consistent developing results.
There are a number of systems that use computer-controlled systems and elaborate algorithms to determine the activity of the developer. For example, EP-A-0 556 690 discloses a computer controlled system for refreshing the partially exhausted developer for those cases where the value of the electrical conductivity is not a measure for the activity of the developer. However, this system requires an expensive computer-controlled system. Therefore, there is a need for a developing process or system that does not require expensive equipment, and does not require skilled workers for its operation. Furthermore, there is a need to reduce the total consumption and the discharge of exhausted printing plate precursor developers.